Use choices for CommunicationEventType.category

From #1435:
CommunicationEventType.category is used also to "detect" order-related
emails, in which case when previewing that email Oscar will put a
user-supplied order in the context for the email template.
This doesn't always work because the string that's saved in category
is localized.

Using choices remedies the problem. I don't think this feature is
commonly used, so no attempt at backwards-compatibility for
non-English setups is made.

South doesn't create a migration for this change.

Fixes #1435.